As the saying goes, "What gets measured, gets managed". In my experience, this is absolutely 100% true...and so is the reverse. If it isn't being measured, the chances are good that it isn't being managed, or at least not managed well.
The real question, and the one that I see so many people stumble on, is this:
"What do I measure?"
Such a simple sounding question, but there are so many possible answers that most people just can't decided what to measure. In such cases, one of two things happens: they try to measure everything and get burned out, or they decide to measure nothing and end up making bad decisions and losing money.
I've been setting goals and tracking data for years, and when it comes to online marketing I've discovered that there is really only one metric that MUST be tracked:
Cost Per Acquisition by Marketing Channel
If you track absolutely nothing else, track this metric. Now, granted, this metric is composed of a number of others like conversion rate, web traffic, average order value and profit margins, but it can be funneled down to that one simple metric.
Think about it, all companies want one of three things: leads, sales, or ad clicks/impressions.
By monitoring your cost per acquisition by marketing channel, you will have the data necessary to use your marketing budget as efficiently as possible, thus generating whatever it is that your company wants at the lowest price possible...and that is what wildly profitable companies are built from.
In addition to business uses, you can also use metrics in your personal life. Let me give you an example from my personal life.
Have you ever thought about resources as renewable vs. non-renewable? Oil, trees, precious metals? What about time?
You see, out of all the things in this world, the only thing that is truly non-renewable is time. You can't ever get it back, you have a finite amount at your disposal, and you never really know just how much time you have. You could run out of time anytime.
As such, I would argue that your time is not only the most valuable thing you possess, but the most valuable thing in the world, and it should have a value commensurate with that understanding.
So, for myself, I've developed a personal metric that I track religiously:
Net Income per Hour Worked
I have a clear list of priorities in my life containing both things I want to do and things I need to do. Spending time with my family, learning new things, traveling and teaching are very high on my list.
By assigning a dollar value to my time, I can then budget enough time to make the money I need to make, while leaving enough time to do the things I really want to be doing.
For example, a salaried worker making $50,000 per year who puts in 80 hour weeks is actually making less per hour than a cashier at Costco. He may have more money, but his time is the non-renewable resource and he should be valuing it higher.
Wealth is determined by cash flow, not by gross income, so approach work with that in mind. And, at the end of the day, if you're miserable from working too much and never have any time to enjoy the fruits of your labors, what was the point?
If you assign values intelligently, measure religiously and make course corrections accordingly, you can have your cake and eat it too :)